1. Field of the Invention
The present invention relates to an eyepiece; and, in particular, to an eyepiece suitably usable for endoscopes.
2. Description of the Prior Art
As an eyepiece in endoscopes, one disclosed in Japanese Unexamined Patent Publication No. 3-59511 has conventionally been known, for example. For making this eyepiece more particularly suitable for endoscopes than are typical eyepieces, it has a greater eyepiece magnification and a longer distance (eye-point distance) from the eyepiece to the pupil position of the viewer. As a consequence, though images have a very small size at the end face of optical fiber bundle in the front end part of an endoscope, it can be observed as a large image which is easy to view, whereby inspections and analyses become easier to carry out. Also, the system of this eyepiece is constituted by three lens elements, so that the eyepiece as a whole can be made smaller with a lighter weight at a lower cost.
Recently, as endoscopes have come into wider use, demands for smaller size, lighter weight, and lower cost have become stronger. In the above-mentioned conventional eyepiece, however, it is very difficult to further reduce the number of lens elements since its performances may be greatly deteriorated thereby in terms of power arrangement and chromatic aberration.
In view of such circumstances, it is an object of the present invention to provide an eyepiece comprising a lens equipped with a diffractive optical surface (hereinafter referred to as diffractive optical element: DOE), which lens has been coming into use in various fields, thereby not only achieving further smaller size, lighter weight, and lower cost, but also being able to correct various kinds of aberration such as chromatic aberration in particular.
The eyepiece in accordance with the present invention comprises first and second lenses, each made of a positive lens having a convex surface directed onto an eye side, at least one of the first and second lenses being formed from a diffractive optical element.
Preferably, in the eyepiece in accordance with the present invention, at least one diffractive optical surface constituting the diffractive optical element satisfies the following conditional expression (1):
E less than 0xe2x80x83xe2x80x83(1)
where E is the coefficient of the quadratic term of the phase difference function.
The eyepiece in accordance with the present invention may be configured such that, among the lens surfaces of the first and second lenses, both of surfaces opposing each other between the first and second lenses are formed as diffractive optical surfaces.